This application claims priority under 35 U.S.C. xc2xa7371 to International Application No. PCT/EP99/01939, filed Mar. 23, 1999, which claims priority to German Patent No. DE 198 14 500.4, filed Apr. 1, 1998.
1. Field of the Invention
This invention relates to a method for the automatic monitoring and control of aqueous process solutions containing nonionic, anionic and/or cationic surfactants. Examples of such process solutions are lyes for the large-scale washing of textiles, cleaning baths for hard surfaces and surfactant-containing iron phosphating solutions. The method is designed in particular for technical process solutions in the metalworking industry, such as in car manufacture. It makes it possible to monitor automatically the functional capacity, as characterised by the parameter xe2x80x9csurfactant contentxe2x80x9d, of the process solution and, if necessary, to supplement the process solution automatically or by external request, or to introduce other bath maintenance measures. The method is in particular so designed that the results of the surfactant determinations are transmitted to a location removed from the process solution. In addition, it is possible to intervene in the automatic measurement procedure from a location removed from the process solution or to initiate repeat metering or other bath maintenance measures. The xe2x80x9clocation removed from the process solutionxe2x80x9d may lie, for example, in a higher-level process control system, in a control room of the works in which the process solution is located, or else at a point outside the works.
2. Description of Related Art
The cleaning of metal parts prior to the processing thereof represents a conventional requirement in the metal-working industry. The metal parts may be contaminated, for example, with pigment soil, dust, metal abrasion, corrosion preventing oils, coolants or mould release agents. Prior to the processing, such as in particular prior to an anti-corrosion treatment (e.g. phosphating, chromating, anodising, reaction with complex fluorides etc.), or prior to a painting, such impurities must be removed by means of a suitable cleaner solution. Spraying, dipping or combined processes are considered for this. If surfactant-containing aqueous process solutions are used for the cleaning, which additionally contain phosphoric acid, a so-called non-film-forming phosphating is carried out simultaneously along with the cleaning. The cleaned metal parts are in so-doing coated simultaneously with a corrosion-proofing amorphous phosphate and/or oxide layer. Processes of this type are used widely in the metal-working industry as combined cleaning and corrosion-proofing processes. When applied to iron-containing materials, they are termed xe2x80x9ciron phosphatingxe2x80x9d.
Non-phosphating industrial cleaners in the metal-working industry are, as a rule, alkaline (pH values about 7 and above, for example from 9 to 12). The basic components are alkalis (alkali metal hydroxides, carbonates, silicates, phosphates, borates), as well as, for the present purposes nonionic, anionic and/or cationic surfactants. The cleaners frequently contain as additional auxiliary components complexing agents (gluconates, polyphosphates, salts of aminocarboxylic acids, such as ethylenediamine tetraacetate or nitrilotriacetate, salts of phosphonic acids, such as salts of hydroxyethane diphosphonic acid, phosphono-butane tricarboxylic acid, or other phosphonic or phosphonocarboxylic acids), anti-corrosive agents, such as salts of carboxylic acids having 6 to 12 carbon atoms, alkanolamines, and foam inhibitors, such as end group-capped alkoxylates of alcohols having 6 to 16 carbon atoms in the alkyl radical. If the cleaner baths do not contain any anionic surfactants, cationic surfactants may be used. The cleaners may in addition contain both nonionic and ionic surfactants.
The cleaners generally contain as nonionic surfactants ethoxylates, propoxylates and/or ethoxylates/propoxylates of alcohols or alkylamines having 6 to 16 carbon atoms in the alkyl radical, which may also be end group-capped. Alkyl sulfates, fatty alcohol ether sulfates and alkyl sulfonates are widely used as anionic surfactants. Alkylbenzene sulfonates are still encountered, but are disadvantageous in environmental terms. There are considered as cationic surfactants, in particular cationic alkyl ammonium compounds having at least one alkyl radical of 8 or more carbon atoms.
It is known in the prior art to determine manually the nonionic surfactants in aqueous process solutions, such as in cleaner baths, by means of a color indicator. The conventional procedure in this case is to add to a sample taken from the process solution a reagent which forms a color complex with nonionic surfactants. Such color complex is preferably extracted into an organic solvent not miscible in all proportions with water and the light absorption thereof then determined photometrically at a particular wavelength. Tetrabromophenolphthalein ethyl ester, for example, may be used as the reagent for forming the color complex. Prior to the extraction into an organic solvent, preferably into a chlorinated hydrocarbon, the process solution is in this case mixed with a buffer system having a pH of 7.
It is further known to determine nonionic surfactants in the presence of ionic surfactants. The ionic surfactants are here separated from the sample by ion exchangers. The nonionic surfactants not bound in the ion exchanger are determined from the refractive index of the process solution leaving the exchanger column.
Anionic and cationic surfactants in aqueous process solutions may be detected, for example, by titration with Hyamin(copyright) 1622 (=N-benzyl-N,N-dimethyl-N-4-(1,1,3,3,-tetramethylbutyl)phenoxyethoxyethylammonium chloride) and potentiometric end-point determination. For this, the sample is mixed with a known quantity of Na-dodecyl sulfate, titration with Hyamin is carried out and the end point of the titration is determined using an ion-sensitive electrode.
Alternatively, anionic surfactants may also be determined by titration with 1,3-didecyl-2-methylimidazolium chloride. An electrode having an ion-sensitive membrane is used as detector. The electrode potential depends on the concentration of the test ions in the process solution.
Depending on the outcome of this surfactant determination involving the deployment of personnel, the operating personnel of the plant supplement the process solution with one or more supplementary components. The procedure thus makes it necessary for operating personnel to be in attendance at the plant site at least during the periods of the surfactant determination. The procedure is personnel-intensive, therefore, in particular in multi-shift operation. The documenting of the results for quality control and quality assurance purposes entails additional expenditure.
Conversely, an object of the present invention is to automate and document the monitoring of process solutions by surfactant determination in such a way that at least the results of the surfactant determination are stored on a data carrier and/or outputted. Preferably the measuring equipment used is itself to be checked and calibrated and an alarm message transmitted to a remote point in the event of a malfunction. Furthermore, it should preferably be possible to check the functional capacity of the measuring equipment and the measuring results from a remote point. It should also be possible to intervene in the measurement procedure and in the maintenance measures for the process solutions from a remote point. The number of personnel deployed on the monitoring and the control of the process solutions is to be reduced by the desired remote control.
This problem is solved by a method for the automatic monitoring and control of the content of surfactants of an aqueous process solution, wherein, under program control:
(a) a sample having a predetermined volume is taken from the aqueous process solution;
(b) if required, the sample is freed of solids;
(c) if required, the sample is diluted with water in a pre-set ratio or one determined as the result of a prior determination;
(d) the content of surfactants is determined by selective adsorption, electrochemically, chromatographically, by splitting into volatile compounds, stripping out of such volatile compounds and detection thereof, or by addition of a reagent which varies the interaction of the sample with electromagnetic radiation in proportion to the content of surfactants, and measurement of the variation of such interaction; and
(e) the outcome of the determination is stored on a data carrier and/or used as a basis for further calculations and/or the outcome of the determination or of the further calculations is transmitted to a remote location.
For present purposes, an aqueous process solution is, in particular, a cleaning solution for hard surfaces, in particular for metal surfaces, or an iron phosphating solution. Process solutions of this type are known diversely in the prior art and are widely used in the metal-working industry.
The sample volume taken in (a) may be programmed permanently into the control section of the measuring equipment to be used for the method. Preferably, the size of the sample volume may be varied from a remote point. The control program may further be created in such a way that it makes the sample volume to be used dependent on the outcome of a previous measurement. For example, a correspondingly greater sample volume may be selected, the smaller is the surfactant content of the process solution. The accuracy of the surfactant determination may be optimized in this way.
When reference is made to a xe2x80x9cremote locationxe2x80x9d in the context of the method according to the present invention, there is meant a location which is not located in direct or at least in visual contact with the process solution. The remote location may, for example, be a central process control system which, as part of an overall process for the surface treatment of the metal parts, monitors and controls a cleaner bath as a subsidiary function. The xe2x80x9cremote locationxe2x80x9d may also be a central control room, from which the overall process is monitored and controlled and which is, for example, located in a different room to the process solution. There is also to be regarded as a xe2x80x9cremote locationxe2x80x9d, however, a point outside the works in which the process solution is located. It becomes possible in this way for the process solution to be monitored and controlled by specialists who are stationed outside the works in which the latter is located. In this way, it is necessary far less frequently for specialist personnel to be stationed at the location of the process solution.
Suitable data lines, with which the results of the surfactant determinations, as well as control commands, may be transmitted, are known.
Between the taking of the sample and the actual measurement, it may be desirable to free the sample of solids in the optional step (b). This is not necessary in the case of a process solution loaded only slightly with solids. If the solids content is excessively high, however, valves of the measurement equipment may become clogged and sensors become dirty. It is recommended, therefore, that solids be removed from the sample. This may take place automatically by filtration or alternatively by use of a cyclone or a centrifuge.